1. MVC is better suited for implementing the sort of separation of concerns that we wanted to implement in Orchard. Plus, ASP.NET WebPages did not exist when we started this work. We are using the Razor view engine though, so the templating language is the
same as in WebPages.

2. We started with TFS/SVN when that was the only option on CodePlex, but immediately switched to Mercurial when that became available because we think that distributed source control works better for open-source projects.

3. When we started this work, EF did not have the code-centric model that we wanted. NHibernate had the best adequation of features with what we were trying to do under a very mature form.

Thank you very much, all is clear except one moment. ASP.net WebPages exists long time, since first .net version. Are we thinking the same when talking about ASP.NET WebPages ?

I think u ment ASP.NET WebForms.
ASP.NET Web Pages is a new lightwight Framework, provided with WebMatrix.
It does not have many features, it's for quickly making web pages and it uses the Razor view engine by default.

Not being the expert on this, my guess is that they wanted to make the platform as accessible as possible for folks building extensions, and there are a lot more C# developers than F# developers... so you get more folks who will look at your source code
and say, "Ah! that's how you do that..." which is great for the plugin / module ecosystem.